The need to measure the conductivity of typically aqueous fluids is ubiquitous throughout various industries such as pharmaceuticals and chemicals, food, metals and mining, steam generation, hydrocarbon processing, textiles and the power sector. Determination of the conductivity of process fluids used in these and other such industries is accomplished by measuring the resistance of the binary or non-binary ionic solution exhibiting the property of conductivity.
One type of EC sensor is a toroidal inductive conductivity sensor. These sensors typically include two toroid transformer coils that are suitably spaced apart. One coil is called a drive coil or transmitter coil, and the other is called a receiver coil or sense coil or a detection coil. When the toroidal conductivity sensor is immersed in a conductive fluid (or the fluid is otherwise disposed within or passed through the toroids) and the drive coil is electrically excited or energized by an alternating current source, the drive coil generates a changing magnetic field.
The changing magnetic field induces a current loop in the sample fluid. The magnitude of the induced current is indicative of the conductivity of the fluid. The current in the fluid in turn induces a current in the receiver coil, which the analyzer measures. The current in the receiver coil is directly proportional to the conductivity of the fluid. This type of invasive toroidal sensor is typically used for conductivity values that exceed 1+ microsiemens/cm.
These EC sensors are also used in conjunction with an analyzer or transmitter that converts the resistance measurements provided by the sensor to actual conductivity values, typically as microsiemens/cm or millisiemens/cm also known as micromhos/cm or millimhos/cm respectively wherein 1 millimho/cm equals 1000 micromho/cm.
The accuracy of this measurement depends in large part on the initial calibration of the specific EC sensor with its associated electronic circuitry. This initial calibration identifies for the electronic circuitry, a low-end conductivity point (typically but not necessarily ‘zero’) and a high range (or full scale) conductivity point, these points corresponding to distinct conductivity values. These conductivity values are each inputted to the electronic circuitry in the form of an input signal corresponding to a specific resistance value. This specific resistance value is determined by the following equation:
                              Resistance          ⁢                                          ⁢          in          ⁢                                          ⁢          ohms                =                                                                                                                        [                      geometric                      ]                                        ⁢                                                                                  ⁢                    cell                    ⁢                                                                                  ⁢                    factor                                    ⁢                                                                                                                                                                                  (                                          of                      ⁢                                                                                          ⁢                      EC                      ⁢                                                                                          ⁢                      sensor                                        )                                    ×                  1000                                                                                                                          Full                  ⁢                                                                          ⁢                  scale                  ⁢                                                                          ⁢                  conductivity                                                                                                      value                  ⁢                                                                          ⁢                  in                  ⁢                                                                          ⁢                                      millisiemens                    /                    cm                                                                                                          (                  Equation          ⁢                                          ⁢          1                )            
A typical current method for calibration is explained below.
A decade resistance box known to a person skilled in the art is used wherein the lead therefrom is passed through the bore of the toroid of the EC sensor, when a specific desired resistance of a particular value (equivalent of a desired conductivity value used in Equation 1) is to be input to the electronic circuit. This specific desired resistance value is then ‘dialed in’ on the decade resistance box and the calibration reading in the electronic circuit is read. Thus the calibration of the sensor for that desired conductivity value is completed.
A primary advantage of this method is that the decade resistance box typically permits the input of a specific desired resistance value by manually adjusting a series of graduated resistance levels on the dials of the resistance box, which adjustment is known to the person skilled in the art. Thus any resistance value that a decade box is capable of providing can be selected for calibration.
There are, however, disadvantages of this method using a decade resistance box. For example, if calibration of the sensor is to be completed away from the shop bench, which is a very common occurrence, then the decade resistance box must be transported to one or more locations, which may be problematic as decade resistance boxes are typically large and cumbersome to transport. In addition, human errors are possible in determining the specific desired resistance value. (For example, an error may occur in using the above mentioned equation or an error may be made in cell factor determination.) Even in the event the correct resistance value is determined, human errors may be introduced by inaccurate input of the desired resistance using the graduated dials of the decade resistance box. In addition, the decade resistance box itself may be ‘out of tolerance’ (for example, due to temperature effects). Moreover, decade resistance boxes typically do not provide precision resistance values, which may result in undesirable inaccuracies.
Alternatively, an EC sensor may be calibrated by use of a single resistor or a pair of resistors mechanically attached to a loop of wire that is passed through the toroidal bore of the sensor. An advantage of this method is that the resistors are lighter and smaller than the decade resistance box. This approach, however, also has disadvantages. For example, human errors are possible in determining the specific desired resistance value (such as in solving the above mentioned equation and/or in determining the particular cell factor). Also, poor quality connections between the resistance wire and the resistor(s) may degrade resistance values and generate inaccurate calibration results. Human errors are also possible in converting resistor color codes.
In view of the above, there is a need to develop a method and apparatus for calibrating an EC sensor, which addresses the above mentioned disadvantages.